In slaughterhouses, after the meat and, in general terms, the useful parts have been cut off, various products, such as the skin, blood, viscera, etc., remain, and these are sometimes referred to, as a whole, as the "fifth quarter". The skin and the blood can be recovered separately, but the other products have to be eliminated, while being exploited as much as possible. However, these products include a considerable proportion of adipose material or, at all events, a material containing a substantial proportion of fatty material. This is usually associated with at least a certain proportion of water within cells limited by membranes of protein tissue, called collagen.
In general, slaughterhouse waste which does not have a particular use contains a considerable proportion of fatty material associated at least with water and with a protein tissue. To exploit these products, they are therefore conventionally subjected to an increase in temperature, making it possible to melt the fatty material in order to separate the melted fat from the residue containing the proteins. Usually, the material is heated in large vats, either by injecting steam or by heating the wall of the vat, and this process, called "dry melting", avoids the need to add to the material a quantity of water which would subsequently have to be eliminated. To achieve a sufficient output, it is customary to process substantial quantities of material in large-size installations. When the melting of the fatty material is completed, it is extracted, for example by means of pressing in a screw press, making it possible to discharge the melted fat by filtration, the residue, called "crackling", being recovered separately.
To date, the installations of this type have operated intermittently and with different appliances each designed for their own particular function. Furthermore, even when the dry-melting process is used, it is necessary to process large quantities of animal material, and the installations cause a great deal of pollution. Consequently, the fat is recovered in separate factories which are sometimes at a great distance from the slaughterhouses, and as a result the animal material first has to be recovered in all the slaughterhouses and then stored for a certain length of time before being processed, but this again entails risks of pollution of all kinds and results, moreover, in a deterioration of the material which makes it necessary to carry out subsequent refining.